To prevent contamination of wafers and generation of natural oxide films, the process of producing semiconductors is generally carried out in a clean room at semiconductor manufacturing plants. Before the air is led into the clean room, some of the chemical components in the air are removed by an activated carbon filter, as necessary depending on the quality of the ambient air. The air is also regulated to be at a constant temperature of 23.degree. C. to 25.degree. C. and a relative humidity (RH) of 40 to 50 percent by an air-conditioning system.
With the advances in the integration of semiconductors, however, it is now clear that the air in the clean room supplied by the air-conditioning system as mentioned is no longer clean enough for the semiconductor manufacturing plants; the chemical components and moisture in the clean room air constitute a new source of contamination for wafers.
In addition to nitrogen, oxygen and noble gases (Ar, Ne, Kr etc. belonging to group O of the periodic table), the air contains diverse chemical components such as hydrocarbons, organic halogens, acidic gases, basic gases, aldehydes, nitrogen oxides, etc., and these diverse chemical components may contaminate semiconductor wafers independently or in the presence of moisture, even if their concentration is extremely low.
The degree of humidity secured as described above is provided to alleviate dryness in the mouths of workers, etc., but when air having this degree of humidity is present, a natural oxide film easily forms on wafers in contact with such air. For example, when the dew point is around -90.degree. C. (moisture content 0.1 ppm), natural oxide film is scarcely formed even after a lapse of some 100 hours, but when the dew point is around 10.degree. C. (moisture content 1.2 vol %), a natural oxide film will form in a few minutes.
Therefore, in order to prevent this kind of wafer contamination and the generation of a natural oxide film, super clean air must be used as the air with which the wafer comes in contact. This super clean air must have chemical components and moisture content removed therefrom to the same degree as super high quality gases such as the nitrogen gas used in the semiconductor manufacturing process. Specifically, in order to prevent wafer contamination, the chemical components in the air must be removed such that the remaining chemical components do not exceed a part per billion (ppb) level, equivalent to the level of chemical components contained in super high quality gases. Also, in order to prevent the generation of a natural oxide film on the wafer, the dew point of the air must be lowered to -40.degree. C. to -120.degree. C. (preferably -100.degree. C. or lower).
The requirement for super clean air causes the production costs to be extremely high as compared to clean room air conditioned and controlled by an air-conditioning system as described above. That makes it too costly and difficult to provide super clean air as the atmosphere in a large volume to fill an entire clean room. Meanwhile, the clean room where wafers come in contact with air has some sections which are originally designed to be of a small volume from the viewpoint of functions and application and also some areas whose volume can be greatly reduced within a range which does not hinder semiconductor production--the wafer transport region between the processes, for example.
Meanwhile, a tunnel type wafer transport system has been proposed which minimizes the volume of super clean air required and at the same time prevents as much as possible the contamination of wafers and the formation of natural oxide films during wafer transport. In this system, the wafer transport region is formed in a tunnel with the minimum required volume for transporting wafers, and super clean air is supplied to the tunnel rather than to the entire clean room. It has also been proposed to supply super clean air to places other than the tunnel (for example, the inner space of a storage apparatus), too, where contact of the wafer with air is likely to permit contamination or formation of a natural oxide film. To prevent wafer contamination, etc. it has also been proposed to supply nitrogen gas to the tunnel, etc., in place of super clean air, but the use of nitrogen is not practical from the viewpoint of cost and safety to humans.
As set forth above, it is possible to effectively prevent wafer contamination and the formation of natural oxide films by supplying super clean air to the wafer transport tunnel, etc. In consideration of costs and other factors, it is desirable that the super clean to be used for the purpose should be produced from atmospheric air.
However, the chemical components in the atmosphere are diverse as described above, and because the types of chemical components contained in material air differ widely depending on where the air is collected from the atmosphere, it is extremely difficult to remove all the chemical components in the material air to the degree that the remaining chemical components are so scarce as to stand at no more than about one ppb. Furthermore, no method has been proposed for producing super clean air efficiently and in large quantities by using atmosphere as the material from which the super clean air is derived. This has hindered the practical applications of the tunnel method to the wafer transport system, etc., described above. Super clear air is needed in not only the semiconductor manufacturing field but also other manufacturing and medical fields where contamination by chemical components in the atmospheric air has to be avoided. And there have been growing calls for development of an efficient method of producing super clear air from the free material atmospheric air.
In answer to such calls, the inventors conducted extensive research and experiments and finally succeeded in developing a method of producing super clean air. This method involves low temperature adsorption treatment of the material air with an adsorbent like synthetic zeolite at a low temperature of not higher than -60.degree. C. In this treatment, the chemical components are reduced down to the one ppb or lower level and the dew point drops to a range from -40.degree. C. to -100.degree. C.
But the formation of a wafer transport path of a tunnel much smaller than the clean room in volume still needs a huge amount of super clean air. To produce such a huge amount of super clean air in the newly developed method, the specific consumption of energy needed to cool the material down to -60.degree. C. or lower will inevitably rise. Because usual refrigerators can not bring the temperature down to -60.degree. C. or lower, liquid nitrogen has to be used. The consumption of this liquid nitrogen is extremely high.
In the new method, it is preferable to adsorb and remove the moisture and carbon dioxide gas by adsorption treatment at room temperature beforehand in order to facilitate the subsequent low temperature adsorption treatment at a temperature of -60.degree. C. or lower and raise the treatment efficiency. In the light of the running cost, it is desired to use, as regeneration treatment gas for the adsorbent, the clean air obtained in the process of producing super clean air by the new method.
The problem is, however, that the treatment requires a large quantity of regeneration treatment gas. If all this comes from the clean air produced in this production process, the yield of super clean air on the material air will naturally decrease substantially, resulting in increased electricity consumption.
Thus, it is essential to bring down the specific energy consumption and to improve the production yield. Unless these questions are solved, the new method is not practicable.
Meanwhile, deep freeze separation type nitrogen production apparatuses are installed at many semiconductor manufacturing plants where super clean air is needed because nitrogen gas is used in large quantities.
Those nitrogen producing apparatuses are generally so designed that the material air is given a pretreatment (in which moisture, carbon dioxide gas etc. are eliminated by adsorption treatment at room temperature), cooled down to the boiling point by a main heat exchanger, then liquefied and rectified at about -170.degree. C. in a low temperature section or a rectifying tower. In that system, produced cooling energy and gas are made good use of. For example, the low temperature gas coming from the rectifying tower through an expander such as expansion turbine is led into the main heat exchanger for cooling the material air, and then utilized as regeneration gas for pretreatment absorbent.
But even such nitrogen producing apparatuses making effective use of cooling energy and gas still leave substantial quantities of cooling energy and gas unutilized. In producing 2,900 Nm.sup.3 /h of nitrogen gas and 100 Nm.sup.3 /h of liquid nitrogen out of 8,250 Nm.sup.3 /h of the material air, for example, the low temperature gas (at -179.degree. C. or so) that passes through the expansion turbine from the rectifying tower amounts to 5,250 Nm.sup.3 /h. The temperature required to bring the material air close to the boiling point by the main heat exchanger is about -170.5.degree. C. The gas sensible heat from this temperature difference can be used which has cooling energy enough for cooling the air to not higher than -60.degree. C. needed in the low temperature adsorption treatment by the newly developed method. It is also pointed out that 5,250 Nm.sup.3 /h of the gas passes through the main heat exchanger, of which 1,250 Nm.sup.3 /h is used as regeneration gas of the pretreatment adsorbent for the nitrogen producing apparatus. The remaining 4,000 Nm.sup.3 /h can be used as regeneration gas for the adsorbent used in the room temperature treatment which is considered as desirable in the newly developed method.
The present invention is based on the discovery that deep freeze separation type nitrogen producing apparatuses are installed at many semiconductor producing plants and that the surplus cooling energy and gas therefrom can be utilized as cooling energy and gas needed in the low temperature adsorbent treatment.